Tunable Diode Laser Absorption Optical Spectroscopy (TDLAS)
2017-12-07

LD-PD INC is working in four areas of optical absorption spectroscopy.

Most optical sensors and analyzers rely on absorption spectroscopy that is often expressed using the Beer-Lamber Law that describes the loss of intensity as a light beam passes through an absorbing sample,

where I0 is the light intensity where the light enters the sample, I is the intensity where the beam exits the sample, n is the absorber number density (or concentration), σ is the absorber cross section which is a fundamental property of the absorber, and L is the length of the optical path through the sample.

Following We test our 2004nm DFB Laser diode as example

Absorbance at 10m,400ppm CO₂ in the air

For all of the work at AOL Labs(Owned by LD-PD INC), the absorbers are gases and the light is generated by a laser.

For most of the gases of interest, the absorption cross section, σ, is available in the open literature or can be measured independently in the lab.  The optical path length is a design parameter of the apparatus.

The product the absorbance, α, which is a dimensionless quantity. Absorber concentration can be determined by measuring the absorbance.  The graph at right shows the absorbance spectrum of water vapor close to 2004nm calculated using the HITRAN spectroscopic database1 for 1 ppm CO₂ in a 1-m-long path.  The absorbance at the strongest peak is 1.4 × 10-4 and if I₀ = 1 mW, then I would be 0.99986.  

Detection sensitivity can be improved by choosing a measurement wavelength where the cross section is large and/or using a long optical path, L.  Detection sensitivity is also determined by the smallest absorbance, αmin , that can be measured reliably.  This is a classic problem in chemical analysis: how best to determine a small change on a large signal.  In absorption spectroscopy, α_min , is defined by the smallest change in I₀, that can be measured.  For laser spectroscopy, the theoretical best is defined by the shot noise limit, 

where C is a Coulomb (6.241 × 10¹⁸ electrons), I is the laser intensity (power) reaching the detector in W, and η is the detector efficiency in A W-1.  Continuing with the water vapor example (1 mW laser power, 1390 nm laser wavelength, 1 m path length) and an InGaAs photodiode detector (η = 0.85 A W^-1),  the shot-noise-limited absorbance, αshot_noise = 1.4 × 10^-8 Hz^-1/2, implying a gas detection limit of 1.4 ppb in a 1 s measurement.  

Detection sensitivities for real-world laser spectrometers are typically 1000× worse due to a variety of noise sources including laser amplitude noise (also called laser excess noise), cross-talk between laser wavelength and amplitude tuning, and – most important of all – unwanted optical interference fringes.  As a result, the “art” of diode laser spectroscopy entails developing techniques to minimize the extra noise or to use long optical path lengths or a combination of the two.  

Long Optical Path Lengths(Long Optical Path Cell)

LD-PD Inc researchers have developed long path lengths in physically compact structures. We use our patterned Matrix structure,The structure is 12-cm-long and contains two squared mirrors.The laser beam enters through a fiber optical collimator and is reflected back-and-forth between the mirrors until the optical pat gets 10m，Light reaches on the Photodiode biult in the cell. A key advantage to Matrix structure cell is the size can designed much smaller than Herriott cells at the same time we can guarantee the most of the original laser power reaches the detector because highly reflective mirrors are readily available. Reflectivity of the coated mirrors in LD-PD’s cells is large enough that the reflective losses vary from 60 to 40% at wavelengths between 1500 nm and 2μm. For most available lasers, particularly readily available, near-infrared lasers, noise far exceeds the shot noise limit. 

Software Control GUI

Directly Absorption Line

2f Signal(Demodulated by Lock-in amplifier)